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| 型號： | AN-15 |
| 英文描述： | TOPSwitch Power Supply Design Techniques for EMI & Safety |
| 中文描述： | 的TOPSwitch電源電磁干擾設(shè)計技術(shù) |
| 文件頁數(shù)： | 30/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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AN-15
A
6/96
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each LISN sense resistor (producing sense voltages with the
same phase) and through the AC input to the power supply. The
output voltage of the in-phase splitter will have balanced
common mode components 6 dB higher than those measured
directly at the LISN because the in phase sense voltages
effectively add. The output voltage of the 180 degree out-of-
phase splitter will have no balanced common mode components
as the sense voltages are now effectively out of phase and
cancel.
Unbalanced Common Mode Splitter Measurement
Unbalanced common mode currents (I
) flow from ground
through either LISN sense resistor. Unbalanced common mode
currents are found when the EMI filter does not have balanced
impedance in each leg or when noise from the power path
returns from the AC mains asymmetrically through one side of
the EMI filter (typically caused by asymmetric parasitic
capacitance). The output voltage of the in-phase splitter will
have unbalanced common mode components equal to those
measured directly at the LISN because there is no cancellation.
The output voltage of the 180 degree out-of-phase splitter will
also have unbalanced common mode components equal to
those measured directly at the LISN for the same reason.
The results of using the two splitters on each of the three types
of emissions is shown in Table 5.
frequency (above which the choke behaves like a capacitor) and
effective Q. Identify multi-resonant behavior due to multiple
layer winding.
EMI Filter Component Measurements
No EMI filter component is perfect. At some frequency all
components “give up” their basic characteristic to the effects of
parasitics.
Measure all capacitors. Identify specifically the self-resonant
frequency (above which the capacitor looks like an inductor)
and effective Q.
Measure all chokes. Identify specifically for the self resonant
Spatial Coupling
As power supplies get smaller the EMI filter gets physically
closer to circuitry acting as noise generators. High dv/dt
voltage waveforms and high di/dt current loops generate fields
which may spatially couple around the EMI filter and induce
emission currents directly in the mains. Noise currents which
couple around the filter must be distinguished from the noise
currents which are passing through the filter.
One way to separate conducted emission currents is to place the
power supply power circuitry and load within a grounded box
as shown in Figure 52. The EMI filter is connected between the
enclosed power supply and AC mains. The box will contain the
fields, allowing conducted emission currents to be directly
measured. This is especially effective when analyzing the
fundamental. (Note that this technique is for investigative
purposes only and must not be used for final test data).
The spatial coupling emissions can be reduced by containing
the fields at their sources with local shields. Local shields over
primary power circuitry such as the flyback transformer, primary
damper, clamp diode, and
TOPSwitch
can be used to contain
fields. Local shields can also be used over secondary circuitry
such as output rectifiers. Shields can also be applied around the
EMI filter although the preferred approach is to contain the field
LISN
OUTPUT
IN
180
°
OUT
OF PHASE
PHASE
Differential (V
D
)
0
V
D
+ 6 dB
Balanced
V
cb
+ 6 dB
0
Common-mode (V
cb
)
Unbalanced
Common-mode (V
cu
)
V
cu
V
cu
Table 5. Splitter Signal Levels.
PI-749-032392
Spectrum
Analyzer
f
d
μ
V
LISN
LISN
GROUNDED BOX
L
N
G
EMI
FILTER
POWER
SUPPLY
&
LOAD
Figure 52. Typical Conducted Emissions System Test Set-Up with
Grounded Box.
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